The use of liposomes for drug delivery has been proposed for a variety of drugs, particularly those which are administered parenterally. Liposomes have the potential for providing controlled "depot" release of the administered drug over an extended time period, and of reducing side effects of the drug, by limiting the concentration of free drug in the bloodstream. Liposomes can also alter the tissue distribution and uptake of drugs, in a therapeutically favorable way, and can increase the convenience of therapy, by allowing less frequent drug administration. Liposome drug delivery systems are reviewed in Poznansky.
Generally, the optimal liposome size for use in parenteral administration is between about 0.1 and 0.3, and up to 0.4, microns. Liposomes in this size range can be sterilized by passage through conventional filters having particle size discrimination of about 0.2 microns. This size range of liposomes also favors biodistribution in certain target organs, such as liver, spleen, and bone marrow (Gabizon), and gives more uniform and predictable drug-release rates and stability in the bloodstream. Liposomes whose sizes are less than about 0.4 microns also show less tendency to agglutinate on storage, and are thus generally safer and less toxic in parenteral use than larger-size liposomes.
A variety of techniques have been proposed for preparing liposomes, including drug-containing liposomes (Szoka 1983). Typically, these methods yield liposomes which are heterodisperse, and predominantly greater than about 1 micron in size. These initial heterodisperse suspensions can be reduced in size and size distribution by a number of known methods. One size-processing method which is suitable for large-scale production is homogenization. Here the initial heterodisperse liposome preparation is pumped under high pressure through a small orifice or reaction chamber. The suspension is usually cycled through the reaction chamber until a desired average size of liposome particles is achieved. A limitation of this method is that the liposome size distribution is typically quite broad and variable, depending on a number of process variables, such as pressure, number of homogenization cycles, and internal temperature. Also, the processed fluid has the potential to pick up metal and oil contaminants from the homogenizer pump, and may be further contaminated by residual chemical agents used to sterilize the pump seals.
Sonication, or ultrasonic irradiation, is another method that is used for reducing liposome sizes. This technique is useful especially for preparing small unilamellar vesicles (SUVs), in the 0.025-0.08 micron size range. However, a narrow size distribution of liposomes can only be achieved at liposome sizes of about 0.05 microns, i.e., when the liposomes have been substantially completely reduced in size. The very small liposomes have limited drug capacity and less favorable biodistribution properties than those in the 0.1-0.4 micron size range, as noted below. The processing capacity of this method is also quite limited, since long-term sonication of relatively small volumes is required. Also, heat build-up during sonication can lead to peroxidative damage to the lipids, and sonic probes shed titanium particles which are potentially quite toxic in vivo.
A third general size-processing method known in the prior art is based on liposome extrusion through uniform pore-size polycarbonate membranes (Szoka 1978). This procedure has advantages over the above homogenization and sonication methods in that a variety of membrane pore sizes are available for producing liposomes in different selected size ranges, and in addition, the size distribution of the liposomes can be made quite narrow, particularly by cycling the material through the selected-size filter several times. Nonetheless, the membrane extrusion method has several drawbacks in large-scale processing. For one, the pores in the membrane tend to clog, particularly when processing concentrated suspensions and/or when the liposome sizes are substantially greater than the membrane pore sizes. The clogged membranes cannot be cleared, because the filter housing configuration does not allow back flushing, and replacing the filter is likely to compromise the sterility of the extrusion operation. Secondly, the membranes themselves are planar disks which must be mounted against a flat mechanical support. This severely restricts the surface area available for extrusion, and leads to slow throughput. Although the problems of clogging and slow throughput can be overcome partially at high extrusion pressures, such requires specially adapted filter holders and membrane tearing become more of a problem. Finally, polycarbonate membranes cannot be steam-sterilized in place, with a high degree of confidence, due to their inherent fragility.